linux-kernel - Re: [PATCH] iommu/arm-smmu: don't unregister on shutdown

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Date:   Wed, 14 Dec 2022 19:34:18 +0200
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     iommu@...ts.linux.dev, Will Deacon <will@...nel.org>,
        Joerg Roedel <joro@...tes.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Michael Walle <michael@...le.cc>,
        Laurentiu Tudor <laurentiu.tudor@....com>,
        Claudiu Manoil <claudiu.manoil@....com>, netdev@...r.kernel.org
Subject: Re: [PATCH] iommu/arm-smmu: don't unregister on shutdown

On Fri, Dec 09, 2022 at 11:24:32AM +0000, Robin Murphy wrote:
> > Fixes: b06c076ea962 ("Revert "iommu/arm-smmu: Make arm-smmu explicitly non-modular"")
> 
> I think that's semantically correct, but I'm pretty sure at that point it
> would have been benign in practice - the observable splat will be a much
> more recent fallout from me changing the iommu_device_unregister() behaviour
> in 57365a04c921 ("iommu: Move bus setup to IOMMU device registration"). The
> assumption therein is that unregister would only happen on probe failure,
> before the IOMMU instance is in use, or on module unload, which would not be
> allowed while active devices still hold module references. I overlooked that
> the SMMU drivers were doing what they do, sorry about that.

Ok, I'll change the Fixes: tag, I didn't notice that iommu_device_unregister()
changed in behavior only later, I just looked at current trees and tried
to infer what went wrong.

> The change itself looks sensible. The point of this shutdown hook is simply
> not to leave active translations in place that might confuse future software
> after reboot/kexec; any housekeeping in the current kernel state is a waste
> of time anyway. Fancy doing the same for SMMUv3 as well?

I can try, but I won't have hardware to test.

Basically the only thing truly relevant for shutdown from arm_smmu_device_remove()
is arm_smmu_device_disable(), would you agree to a patch which changes
things as below?

diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
index 6d5df91c5c46..d4d8bfee9feb 100644
--- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
+++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
@@ -3854,7 +3854,9 @@ static int arm_smmu_device_remove(struct platform_device *pdev)
 
 static void arm_smmu_device_shutdown(struct platform_device *pdev)
 {
-	arm_smmu_device_remove(pdev);
+	struct arm_smmu_device *smmu = platform_get_drvdata(pdev);
+
+	arm_smmu_device_disable(smmu);
 }
 
 static const struct of_device_id arm_smmu_of_match[] = {